The wellhead and tree are frequently the top most components of subterranean wells. The wellhead can be used to provide several connections and access ports. A typical connection will be to a pipe conduit for transport of produced fluids away from the well. A top entrance port is also generally provided for allowing access into the well from above. The access port will be used for entering the well during completion and production phases of the well, but must be reliably closed when not in use. The wellhead plug provides such a closure when locked in sealing engagement within the access portion of the wellhead. The length of the segment of the wellhead in which the plug is lodged is partially dependent upon the length of the plug itself. The wellhead plugs described herein will typically be used in 37 horizontal" and SPOOLTREE wellheads.
The materials used in the construction of a wellhead are of high-quality and therefore expensive. The high cost is warranted because the wellhead provides a barrier between potentially polluting fluids carried within the well and the environment. The expensive nature of the constituent materials of the wellhead and the fact that a segment of the wellhead is dependent upon the length of the plug precipitates a common goal in wellhead plug design which is minimizing the length of the plug to reduce construction costs of the wellheads. This is particularly true when more than one plug is used within a wellhead for sealing redundancy; in that case, the savings will multiply.
Referring to FIG. 3, a known wellhead plug is illustrated. While it has been desirable to have such wellhead plugs as short as possible, typical embodiments of the illustrated version still have lengths exceeding twelve inches. The shortest known example of the illustrated wellhead plug exceeds eleven inches, regardless of the number and type of sealing means utilized. The difficulty to further decrease the length of the plug is due to construction features embodied therein.
A lengthening feature of the wellhead of FIG. 3 is the arrangement in longitudinal series of several of the wellhead plug's construction characteristics. As an example, the housing/cap thread connection is longitudinally positioned between the seal and anchor means so that the entire length of the thread connection contributes directly to the overall length of the plug assembly. A further detriment of the illustrated configuration stems from the fact that the length of the thread connection increases as the diameter of the plug increases. The increase in thread length is required to provide sufficient structural soundness to the assembly. Greater forces are applied to larger plugs because of the increased area upon which pressures act. If the thread connection is not lengthened, there is a risk that the structural integrity of the plug assembly will falter; that is, the mating threads may strip out and the connection separate. An additional disadvantage of directly increasing the overall length of the plug with elongations of the thread connection is that plugs for different diameter wellheads will have different lengths and lack desired uniformity which increases design time and costs.
In the illustrated configuration of FIG. 3, the length of the plug assembly increases by an amount equal to the distance over which relative motion is required between the expander sleeve and retainer housing. Like the thread connection, this results because the motion limiting connection is in series with the anchor and seal means.
Operational drawbacks may also be encountered in plugs made according to the example of FIG. 3 wherein the expander sleeve is retained within the plug body by a retaining lip positioned at the upper end of a top extension of the key retainer housing. The lip creates a restraining cavity that may become fouled by solid debris that inhibits relative motion between the expander sleeve and the plug body.
In summary, it has been appreciated for some time that a primary goal of wellhead plug design is to shorten the overall length of the plug; in view of known designs, the lower limit has previously exceeded eleven inches.